The present invention relates to stainless steels and in particular to 13-8Mo steels having significantly improved fracture toughness (K.sub.IC) over conventional 13-8Mo steels.
It is well known to those skilled in the art that fracture toughness is a measure of a material's resistance to crack propagation and catastrophic failure and is an important characteristic in the design of certain critical components. Generally for metallic alloys, toughness is inversely related to strength, i.e. the higher the strength, the lower the toughness. Within this general relationship, individual alloys and families of alloys display distinctive relationships between strength and toughness. These characteristics can be clearly seen in FIG. 1. Precipitation hardening (PH) stainless steels, as a group, tend to be found in the less desirable, low strength, low toughness portion of this figure.
It is generally well known that small amounts of certain elements or impurities, including metallics, metalloids or non-metallics, can dramatically alter the properties of all alloys. The specific elements or impurities and the amounts which result in harmful effects vary widely, depending upon the alloy, the condition and the properties of interest. For example, in 13-8Mo steels as described in U.S. Pat. No. 3,556,776 to Clarke et al. which is hereby incorporated in its entirety by reference, critically low levels of manganese, silicon, phosphorus, sulfur and nitrogen resulted in good ductility in combination with great strength.